Electrical wire has been used in a wide variety of applications. In many applications the conductor is surrounded by an electrically insulating thermoplastic covering. While many of the requirements for the insulating thermoplastic covering vary with how and where the electrical wire will be used, most applications, particularly high voltage applications such as automotive underhood applications, require that the insulating thermoplastic covering be free of spark leaks. Spark leaks are caused by imperfections, such as pinholes, in the insulating covering surrounding the wire. In the production of electrical wire for automotive applications the electrical wire is tested for spark leaks and when a spark leak is found the wire is cut and the section containing the spark leak is discarded. The presence of spark leaks during manufacture interrupts the continuity of the wire and decreases productivity. Because the wire is cut to remove the section containing the spark leak multiple lengths of wire result. These lengths are typically combined to form an overall total length that is packaged and sold.
Electrical wire is typically sold on spools or in containers containing a total amount of wire length determined in part by the cross-sectional area of the conductor. The electrical wire is removed from the spool or container for use in various articles such as automotive wiring harnesses. For example, an electrical wire having a conductor cross-sectional area of 0.14 square millimeters to 1.00 square millimeters, the total length of wire on the spool can be 13,500 to 15,500 meters and the number of individual wires on the spools can be 1 to 6 wherein the minimum length of each wire is 150 meters. Spools or containers containing a larger number of individual wires or shorter lengths of wire often result in lower productivity and higher yield losses in the manufacture of the articles from the electrical wire.
Automotive electrical wire located under the hood in the engine compartment has traditionally been insulated with a single layer of high temperature insulation that is disposed over an uncoated copper-wire conductor. Thermoplastic polyesters, cross linked polyethylene and halogenated resins such as fluoropolymers and polyvinyl chloride have long filled the needs in this challenging environment for heat resistance, chemical resistance, flame retardance and flexibility in the high temperature insulation.
Thermoplastic polyester insulation layers have outstanding resistance to gas and oil, are mechanically tough and resistant to copper catalyzed degradation but can fail prematurely due to hydrolysis. The insulation layer(s) in thermoplastic polyester insulated electrical wires have also been found to crack when exposed to hot salty water and have failed when subjected to humidity temperature cycling.
There is an increasing desire to reduce or eliminate the use of halogenated resins in insulating layers due to their negative impact on the environment. In fact, many countries are beginning to mandate a decrease in the use of halogenated materials. However, as much of the wire coating extrusion equipment was created based upon the specifications of halogenated resins such as polyvinyl chloride, any replacement materials must be capable of being handled in a manner similar to polyvinyl chloride.
Cross linked polyethylene has largely been successful in providing high temperature insulation but this success may be difficult to sustain as the requirements for automotive electrical wire evolve. The amount of wiring in automobiles has increased as more electronics are being used in modern vehicles. The dramatic increase in wiring has motivated automobile manufacturers to reduce overall wire diameter by specifying reduced insulation layer thicknesses and specifying smaller conductor sizes. For example, ISO 6722 specifies, for a conductor having a cross sectional area of 2.5 square millimeters, that the thin wall insulation thickness be 0.35 millimeters and the ultra thin wall insulation thickness be 0.25 millimeters.
The reductions in insulation wall thicknesses pose difficulties when using crosslinked polyethylene. For crosslinked polyethylene the thinner insulation layer thicknesses result in shorter thermal life, when aged at oven temperatures between 150° C. and 180° C. This limits their thermal rating. For example, an electrical wire having a copper conductor with an adjacent crosslinked polyethylene insulation layer having a 0.75 millimeter wall thickness is flexible and the insulation layer does not crack when bent around a mandrel after being exposed to 150° C. for 3,000 hours. But in a similar electrical wire having a crosslinked polyethylene insulation layer having a 0.25 millimeter wall thickness the insulation layer becomes brittle after being exposed to 150° C. for 3,000 hours. The deleterious effects created by these extremely thin wall requirements have been attributed to copper catalyzed degradation, which is widely recognized as a problem in the industry.
Accordingly, there exists a need for covering materials suitable for use in conjunction with conductors, particularly in an electrical wire suitable for use in an automotive environment and is free of halogenated resins.